Mill blocks are typically polygonal, in many cases cylindric or cuboid moulded specimen. They can be prepared by filling a curable formulation into a mould and subsequent curing of the material to give specimen of sufficient mechanical strength for milling or grinding. The cured specimen can optionally be cut into the desired shape and are often fixed on a sample holder for use in milling machines. Radical curing of such materials is often accompanied with undesirable high shrinkage of the specimen. As a result, those materials have high internal stress, which is often relieved by cracking of the specimen or by fraction or chipping during milling or during the usage in the oral cavity.
Composite miling blocks (CMBs) can also be used in prosthetic dentistry. Todays milling blocks in terms of curing chemistry are often based on (meth)acrylate technology. That means the polymerizable resins used in the formulation comprise esters of acrylic and/or methacrylic acid. CMBs are obtained by curing such formulations radically either by radiation activated generation of radicals or by thermal or redox generation of radicals. CMBs are often used for the production of e.g. crowns, inlays, onlays or partial crowns. To obtain these, CMBs are milled in computer controlled milling/grinding machines.
U.S. Pat. No. 4,937,928 (van der Zel) describes a method of making a dental crown by means of a CAD-CAM system, wherein under the control of the CAD-CAM system a model of the dental preparation is made from a certain material, to which model successively two or more layers of another suitable material are applied.
U.S. Pat. No. 7,086,863 B2 (van der Zel) relates to a method of producing an artifical tooth comprising a base layer and a cover layer. The method comprises a variety of steps including the steps of constructing the base layer and applying a cover layer over the base layer.
WO 2010/110662 A2 (Voit et al.) describes a method for manufacturing a dental restoration comprising a dental restoration core and a veneer layer, comprising: providing the material of the dental restoration core with a base colour, providing the constituent material of the veneer layer with a surface colour, the constituent material of the veneer layer being translucent below an optical thickness.
U.S. Pat. No. 5,151,044 (Rotsaert) relates to a blank for the manufacture of artificial teeth and crowns, which comprises a layered block of synthetic plastics materials, the layers being of predetermined varying colour, chroma, value and translucency.
U.S. Pat. No. 6,994,549 (Brodkin et al.) describes the use of solid free form fabrication techniques for creating dental restorations.
U.S. Pat. No. 7,981,531 (Rheinberger et al.) describes a multi-coloured shaped body having layers arranged on top of one another for producing dental restorations.
US 2010/0244294 A1 (Karim et al.) relates to a dental article as a crown being fabricated by layering one or more preformed shells of SMC material onto an understructure.
EP 1 561 433 A1 (Cadent Ltd.) describes a method and system for manufacturing a dental prosthesis. Dimensional data of at least one critical parameter of the workpiece from which the crown is to be manufactured is determined and compared with corresponding target dimensional data of the desired prosthesis.
US 2010/0310786 A1 (Dunne) relates to systems and methods employing various modes of material deposition and material removal to build a complex, three-dimensional structure upon a rotating base.
EP 0 850 601 B1 describes a semi-finished dental part wherein the moulding is constructed from the core and at least one layer with different optical properties.
These systems often possess limitations regarding productivity, aesthetics or precision.
Thus, there is still a need for improvement, especially with respect to processes and milling blocks which can be used for producing dental restorations.